Trp repressor protein of Escherichia coli is an alpha 2 homodimer (M/r per monomer, 12,339) that forms stable noncovalent complexes with a number of operator targets in double-stranded DNA, provided that certain conformational changes, mediated by L-tryptophan, have first taken place. Using the techniques of molecular genetics, protein chemistry, in vitro segment-directed mutagenesis and NMR specifrometry, we will seek to further refine our knowledge of which amino acids within Trp repressor are important for dimerization, ligand binding, and operator recognition. Particular attention will be focused on helix D, an eight amino acid stretch that comprises one part of a "helix- turn-helix motif" common to all operator-binding proteins. The trpR gene is one member of a group of a least eight genes that bear significant structural homology to one another, as determine in Southern blotting experiments with total genomic DNA. The remaining members of this gene family will be cloned and structurally characterized. Several observations point to the existence of post- transcriptional and/or post-translational mechanisms for controlling the intracellular levels of functionally active Trp repressor. The details of these mechanisms will be examined using biochemical and genetic approaches, with a view to clarifying the broader aspects of how cells maintain regulatory specificity by controlling the levels of proteins like Trp repressor that have the capacity to intrude in non-physiological ways into heterologous systems. This is an important issue, because when strains are engineered so as to produced greatly elevated levels of Trp repressor, one observes reduced expression of the phe, leu, ilv and thr operons. This repression has been traced to the ability of Trp repressor to bind with low affinity to trpO-like sequences lying within the promoters for the above-named operons.